1. Field of the Invention
The present invention relates to a photometric circuit for incorporation into a photographic flash device of the type in which the amount of light emitted is automatically adjustable.
2. Background Art
A photographic flash device of the type which automatically adjusts the amount of light emitted is known. Such automatic light emission adjusting flash devices comprise a light emission stopping circuit to stop light emission from the flash discharge tube and a photometric circuit to apply to the light emission stopping circuit a photometric signal, thereby to activate the light emission stopping circuit in accordance with the intensity of light reflected from a subject being photographed.
Many light emission stopping circuits are either of the serial control or the parallel control types. In a serial control light emission stopping circuit, a switching member is connected in series with a flash discharge tube and is turned off when the photometric signal interrupts current to the flash discharge tube, stopping light emission therefrom. In light emission stopping circuits of the parallel control type the switching member is connected in parallel to the flash discharge tube and is turned on by the photometric signal short circuiting the flash discharge tube to stop light emission therefrom.
The photometric circuit usually comprises a photoconductive element which is exposed to light emitted from the flash discharge tube and reflected from the subject being photographed. The photometric circuit photoelectrically converts the light thus received into an electrical signal which is integrated by an integrator. A switching circuit is provided which is activated when the integrated electrical signal reaches a predetermined level.
To protect the photometric circuit against erroneous activation due, for example, to flash emission from the flash device of another photographer, the photometric circuit is adapted to be energized only in synchronization with the start of light emission from the flash discharge tube with which it is associated. This is achieved in known photographic flash devices, for example, by a discharge current from a capacitor which flows through the flash discharge tube as it begins to emit light. The discharge current is utilized to energize the photometric circuit. In an alternative known device the photometric circuit is energized responsive to activation of a trigger circuit which starts light emission from the flash discharge tube.
In the known devices described above the photometric circuit is coupled to the flash discharge tube or to the trigger circuit. Thus the photometric circuit is necessarily incorporated into the high-tension side of the DC--DC converter utilized in such flash devices. As a consequence, the photometric circuit must be at least partially a high-tension circuit in which high voltage withstanding parts or elements are employed, making realization of the photometric circuit in an integrated circuit form difficult.